a) Field of the Invention
This invention relates to control apparatus for a door-locking mechanism including an electrically-released latch for locking an associated door closed. In particular, this invention concerns industrial doors, guards, screens and so on, to protect industrial machinery or other processes which could be hazardous to operators.
b) Description of the Related Art
With increasing emphasis on the health and safety of employees, it is becoming ever more important to provide competent and reliable protection for industrial machinery or equipment operating processes which are hazardous to the personnel in the vicinity of that machinery. Most such machinery now is fitted with various kinds of doors, guards, screens or other equipment which restricts access to the machinery or the process being performed by the machinery, until the machinery has been made safe. All of such doors, guards, screens and the like and which are moveable between closed and open positions, respectively where access is restricted and access may be gained to industrial machinery or processes, are referred to hereinafter simply as “doors” irrespective of the precise nature of the door, guard, screen or the like.
A simple machine may be protected with a guard structure having a door moveable between the closed and open positions, as referred to above. With more complex machines, it is usually necessary to provide a number of such doors, all of which are interlinked in some way so as to ensure access can be gained only when it is safe to do so. Further, with yet more complex industrial processes, it may be necessary to interlink guards associated with different machines or equipment, such that in the event access is gained at one location to a process, then other action is taken to ensure safety at all protected locations.
In order to accommodate such complex machines and processes, it has been the conventional procedure to provide each door with an electrically-operated lock to hold the door closed, the lock being electrically-released only when it is safe to do so and so when the machinery or process being protected is in a safe condition. Typically, respective sensors are provided for all of the doors and the outputs from those sensors are used to inhibit restarting the machinery or process until all of the doors are in their closed positions.
As the complexity has increased, it has become more and more difficult to ensure complete safety, having regard to the number of sensors which must be provided, together with all of the wiring associated with those sensors and the electrically-operated locks. In an attempt to solve this problem, it is known to provide a bus system which uses relatively few wires (and typically only two or three) between all of the sensors and the locks, on which bus system appropriate control signals are carried to and from a computerised controller, running an appropriate program for operation of the machinery or industrial process as well as control of the doors protecting that machinery or process.
Such bus systems as described above are well known and are largely very effective. However, it is possible for a determined operator to gain access to machinery or industrial process by not properly closing a door and deliberately operating the associated door sensor (which typically is a mechanical switch) to simulate door closing. Further, and bearing in mind the electrically noisy environment within which such bus systems frequently have to operate, it is possible for false signals to be generated which permit restarting of machinery or which do not properly shut down machinery when that machinery is not properly guarded.
It is a principal aim of the present invention to address the above problem, and to increase the reliability and safety of a known design of centrally-controlled bus system for door locking, in a very simply and effective manner and without introducing extra control software or wiring.